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Pasture intake of grazing livestock needs to be estimated to allow determination of energy and nutrient intakes. It is commonly estimated by difference, subtracting intakes of other feeds from estimated needs for dry matter or energy. However, these estimates are often erroneous, because they do not take individual animal variation for growth, reproductive status or activity level into account. One method that has had success in grazing ruminants has been the use of markers, or tracers, to estimate fecal output and nutrient digestibility. External markers are dosed to the animal and can be used to determine fecal output. Internal markers are an inherent part of the diet in question and can be used to determine dry matter and nutrient digestibilities. These estimates can then be used to give estimates of intake. These studies were conducted to evaluate the effectiveness of traditional marker methods in determining fecal output, digestibility, and thus intake in grazing horses. The first trial was conducted on 8 mature mares and geldings, housed in stalls, to determine if a common external marker, Cr, could be used to determine fecal output. Horses were dosed once daily with a molasses, Cr, and hay mixture for 12 d. Feces were collected throughout the day into individual tubs so that total fecal output (TC) could be measured. Daily fecal Cr excretion values (Ct, mg/kg DM) were fit to a monoexponential equation with one rate constant (k), rising to an asymptote (Ca): Ct = Ca - Ca.e-kt. Superior fits were found when a delay (d) was incorporated into the equation, estimating the time required for Cr to enter the prefecal pool: Ct = Ca - Ca.e-k(t-d). Estimates of fecal output (FO) were calculated using the equation: FO = Cr dose-d / Ca and provided good estimates when compared to TC values. Subsequent trials evaluated to use of internal markers and more frequent dosing of Cr to improve estimates of intake. Eight mature geldings were housed in stalls and were fed 2 hays in a replicated Latin Square design. The monoexponential equation with the delay continued to fit the data well. Thrice daily dosing of Cr improved the predictions of FO, when dosing was every 8 h. The internal marker, yttrium (Y) consistently overestimated digestibility (D). The internal markers, n-alkanes, gave a better estimate of digestibility. When the digestibility estimates were combined with the FO estimates to estimate dry matter intake (DMI, kg/d): DMI = [FO / (1-D)]*100, the combination including n-alkanes gave better estimates. Further studies found that dosing Cr for 12 d did not improve the fit of the monoexponential equation compared to dosing for only 8 d. Marker methods that had been developed in stalls were applied to grazing horses, and results continued to be promising. / Ph. D.

Glucose and lipid metabolism are dysregulated in obese horses. Altered glucose metabolism is evidenced by the development of insulin resistance and increased fasting plasma insulin concentrations (hyperinsulinemia) while altered lipid metabolism is evidenced by increased plasma lipid concentrations. Obesity in horses also increases the risk of the painful hoof disease, laminitis. Three experiments were performed to investigate the regulation of nutrient metabolism in skeletal muscle and adipose tissue of lean, healthy horses. Adipose tissue was found to be the primary lipogenic tissue of horses, with acetate being the primary lipogenic substrate. Secondly, ten, lean horses were used to investigate the effects of acute hyperinsulinemia on nutrient metabolism. Increasing plasma insulin concentrations to >1,000 mIU/L for six hours decreased transcript abundance of glucose transporters and the insulin receptor in adipose tissue, and decreased protein abundance of the insulin receptor in skeletal muscle, potentially indicating that hyperinsulinemia potentiates insulin resistance. Insulin infusion also reduced mRNA abundance of lipid transporters in adipose tissue while increasing them in skeletal muscle. The final experiment investigated the influence of the insulin-sensitizing drug, pioglitazone, and lipopolysaccharide, on nutrient metabolism in skeletal muscle and adipose tissue, and their association with insulin sensitivity. Pioglitazone treatment did not increase insulin sensitivity; however it did increase skeletal muscle transcript abundance of the insulin receptor and the non-insulin sensitive glucose transporter and adipose tissue protein abundance of the insulin-sensitive glucose transporter (GLUT4). Lipopolysaccharide decreased insulin sensitivity regardless of pioglitazone pre-treatment, which was associated with decreased transcript abundance of GLUT4 in skeletal muscle and adipose tissue of untreated horses, but not adipose tissue of pioglitazone treated horses. / Ph. D.

The objective of the study was to investigate and characterize the effects of a continuous rate infusion of dextrose or dextrose and insulin on glucose and insulin dynamics in both healthy and endotoxin-exposed horses. Administration of a low dose of endotoxin has been used in horses to mimic the clinicopathologic changes seen in endotoxemia, including the development of an inflammatory response. Our hypothesis was that a continuous rate infusion of insulin at a rate of 0.07 IU/kg/hr would prevent the development of hyperglycemia induced by administration of dextrose in both healthy and endotoxin-exposed horses. Nine healthy adult horses were used in the study. In Phase 1 of the experiment, horses received a saline infusion or a dextrose infusion in a balanced crossover design. In Phase 2 of the experiment, horses received a dextrose and insulin infusion, both prior to and after receiving a low dose of endotoxin (no LPS group and LPS group respectively) in a balanced crossover design. Blood samples were collected at regular intervals throughout both phases for measurement of plasma glucose and insulin concentrations. Infusion of dextrose alone resulted in hyperglycemia for nearly the entire study period. Insulin concentration was also increased in comparison to the saline infusion. When comparing the dextrose treatment group to the combined dextrose and insulin treatment group (no LPS group), the insulin levels were significantly greater over time in the latter group and resulted in maintenance of euglycemia. When comparing the no LPS group to the LPS group, both the glucose and insulin concentrations were higher in the LPS group but euglycemia was still achieved. These results serve to validate the dose of insulin used in this study (0.07 IU/kg/hr) in regards to effective prevention of hyperglycemia when administered concurrently with a dextrose infusion. Hyperglycemia was prevented in both healthy and endotoxin-exposed horses. In addition, the dose of insulin used was demonstrated to be safe, as hypoglycemia did not occur in any of the horses. / Master of Science

This study investigated effects of diet and probiotic supplement on stress in Thoroughbred foals at weaning. Twenty foals, whose dams were paired by age and breeding date, then randomly assigned to one of two diets prior to parturition, were used. Two groups were maintained on mixed grass pastures and fed supplements, one high in sugar and starch (SS) and one high in fat and fiber (FF) that met or exceeded NRC requirements. Half the foals on each diet were fed a commercial probiotic (Probios) containing lactic acid bacteria, while the rest were given a placebo. Plasma, fecal samples and behavioral ethograms were collected for four days pre- and post-weaning, and an ACTH response test was administered 48 h post-weaning. Cortisol, lactate, IgG, IgA and a-tocopherol were analyzed in plasma, volatile fatty acids and pH in feces. Foals fed FF had higher concentrations of IgA (P = 0.006), IgG (P = 0.012) and a-tocopherol (P = 0.005). Butyric and valeric acid concentrations were higher in feces of SS but not FF foals (P = 0.052), which may reflect better adaptation to forage in FF foals. Foals supplemented with probiotic had higher fecal lactate (P = 0.002) and lower fecal acetate (P = 0.0003) concentrations, suggesting that the lactic acid bacteria survived to the hindgut. Probiotic supplementation did not appear to benefit foals at weaning. Supplementation with FF may improve immune status and encourage a more diverse intestinal microbial population, enabling foals to better cope with the physiological stresses of weaning. / Master of Science

Suspensory desmitis is a common problem and affects a broad cross section of equine athletes in various disciplines. For this study, the proximal portion of the suspensory ligament was collected from 6 horses without suspensory ligament injury (16 ligaments) and 4 horses with degeneration of the suspensory ligament (11 ligaments). Specimens were collected immediately after euthanasia and placed in neutral-buffered 10% formalin. The tissue was fixed, sectioned, and stained with hematoxylin and eosin (H&E), Masson's trichrome, and for apoptosis by the terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) technique. Histological changes in the abnormal ligaments included mineralization, fibroplasias, neovascularization, collagen degeneration, and significant architecture disruption in 2 ligaments. There was a trend for increased apoptosis in the injured ligaments compared to the normal ligaments. / Master of Science

Two studies were conducted on endurance horses predominantly of Arabian breeding participating in an 80 km ride dedicated to research in April 2001 (Trial 1) and April 2002 (Trial 2). Objectives were to determine effects of endurance exercise, antioxidant supplementation, and a feed rich in fiber and fat vs. a high fat sweet feed on immunoglobulin A and G concentrations as well as identify selective IgA deficiency in endurance horses of Arabian breeding. There were no effects of distance in Trial 1 on IgA (P = 0.73) or IgG (P = 0.18) concentrations. In Trial 2, IgA concentrations increased (P = 0.05) and IgG concentrations increased (P = 0.006) after the start of the race. There were no effects of antioxidant supplementation on IgA (P = 0.16), IgG (P = 0.16), and IgM (P = 0.70) concentrations. There were no diet effects on IgA (P = 0.80), IgG (P = 0.59), and IgM (P = 0.54) concentrations. There were horses in both trials that were deficient in IgA only. Concentrations of IgG and IgM were within normal ranges, and there were no differences in training, performance and transportation variables, IgG concentrations, antioxidant supplementation, and feed supplementation compared to the horses with normal IgA concentrations. The concentration of IgM was higher in IgA deficient horses in Trial 1 (P = 0.035) and Trial 2 (P = 0.017). Horses with deficient IgA tended to be associated with health problems commonly found in humans and dogs affected with selective IgA deficiency. / Master of Science

Justification: Endotoxemia and sepsis are major causes of mortality in horses, resulting in significant economic losses for the equine industry. Objective: To determine the effects of the combination of Hypertonic Saline Solution and Hetastarch in endotoxemic horses. Animals: Eighteen horses divided into three groups of six. Procedure: All horses received a total dose of intravenous E. coli endotoxin infused at 50 ug/kg; divided into a bolus infusion of 20 ug/kg followed by 30 ug/kg given over 30 minutes. After induction of endotoxic shock; group I (control) received a bolus (15 ml/kg) of isotonic solution, group II (isotonic solution) received a bolus (60 ml/kg) of balanced polyionic crystalloid solution and group III (Hypertonic saline plus Hetastarch) received a bolus of 5 ml/kg of hypertonic saline, followed by a bolus of 10 ml/kg of Hetastarch. Hemodynamic and hematological parameters were measure at different time points. Results: Hemodynamic, biochemical and hematological differences were observed among the three groups. Conclusions and Relevance: the use of large volume crystalloid fluid resuscitation causes volume overload, exerting deleterious effects on the cardiovascular and pulmonary systems. The use of small volume resuscitation (HSS-HES) showed a trend towards better cardiovascular and pulmonary function, without the deleterious effects of volume overload. Abnormalities with regard to coagulation were not seen for the time period of the experimental protocol and the dose regimen used for HSS-HES. Small volume resuscitation in critically ill horses shows promise for its beneficial effects in cardiovascular and pulmonary functions. / Master of Science

Aerosolized bronchodilator drugs are commonly used for treatment of horses with chronic obstructive pulmonary disease. Relative efficacy of sympathomimetic and parasympatholytic bronchodilators for relief of acute airway obstruction and improvement of pulmonary gas exchange was compared in 6 horses with COPD. Physical examination, arterial and venous blood gas analysis and measurement of end-tidal CO₂ tension were performed at time zero, 30 minutes, 1, 2, 3 and 4 hours after administration of aerosol ipratropium (0.35 μg/kg), albuterol (1 μg/kg) or placebo via an equine Aeromask and metered dose inhaler. Physiologic shunt fraction (QS/QT), alveolar dead space fraction (VD/VT) and alveolar to arterial oxygen tension difference (p(A-a)O₂) were calculated using standard formulas. At time zero, horses demonstrated severe respiratory compromise and marked alterations in pulmonary gas exchange, indicative of alveolar hypoventilation, VA/Q mismatching and diffusion impairment. Ipratropium treatment significantly (p< 0.05) reduced arterial CO₂ tension and end-tidal CO₂ tension toward normal, but significantly increased p(A-a)O₂ from baseline. The change in paCO₂ after ipratropium treatment was significantly different from albuterol and placebo treatment groups. There were no significant changes in response variables after albuterol and there were no treatment by time interactions. These results indicate that, under the conditions of this study, ipratropium (0.35 μg/kg) improved alveolar ventilation and had superior bronchodilator efficacy than albuterol (1 μg/kg) in horses with severe COPD. Marked impairment of pulmonary gas exchange persisted after bronchodilators, emphasizing that anti-inflammatory therapy and environmental control are also necessary for effective treatment of severe equine COPD. / Master of Science

Estimation of the glomerular filtration rate (GFR), accepted as one of the earliest and most sensitive indicators of renal dysfunction, can be determined in horses using standard techniques In adult horses and foals, renal dysfunction can occur as a secondary complication to gastrointestinal disorders, dehydration, septicemia, endotoxemia and nephrotoxic drug administration. Measurement of renal function is an important feature not only in the diagnosis, but also in the prognosis and management of renal disease. Commonly used drugs such as phenylbutazone and gentamicin can be highly nephrotoxic under certain conditions. Of particular concern are those drugs, including the aminoglycoside antibiotics, that are eliminated almost exclusively by the kidney. Knowledge of a patients renal status prior to treatment would direct efforts at; 1) restoring kidney function prior to protracted therapy with potentially damaging drugs, 2) adjusting the dose of a life-saving drug based on the magnitude of dysfunction, or 3) selecting a drug that is not dependant on renal function for elimination. such as endogenous or exogenous renal creatinine clearance. Unfortunately, these techniques can be time consuming, dangerous to perform on fractious patients, require trained personnel and are subject to errors most often associated with improper or incomplete urine collection. Recently, tests using iohexol, a radiographic contrast agent, have been developed to estimate the GFR in human beings, dogs and cats with results that have been validated by traditional standards. Most testing protocols require a single bolus injection of iohexol, followed by 2 or 3 blood samples obtained over a few hours. Compared to traditional testing methods, samples are easily and rapidly obtained making the testing procedure less stressful for the patient. A simple method to measure GFR in horses that does not require urine collection, would allow veterinarians in a clinical setting the ability to determine a patient's renal status easily and safely. The objectives of this study were; 1) model the pharmacokinetic profile of iohexol in horse foals, 2) compare creatinine clearance, an accepted standard for GFR determination in patients, with iohexol clearance, and 3) develop sampling parameters and calculation methods for a practical test, based on iohexol clearance, that compares favorably with creatinine clearance in horse foals. Iohexol concentration time data were best described using a 3-compartment open model. Mean creatinine clearance (2.17 ml/min/kg) and mean iohexol clearance (2.15 ml/min/kg) showed good agreement. In addition, GFR values for all foals using either method were within published reference ranges for this species. The results of this study indicate that a single intravenous injection of iohexol at a dose of 150 mg/kg, followed by collection of 2 serum samples at 4 and 6 hours post injection can be used to estimate the GFR in healthy horse foals. Mean corrected GFR value (CLpredicted) for 10 foals in this study was 2.15 ml/min/kg. / Master of Science